1. Field of the Invention
The present invention relates to a sheet supply unit for use in an image forming device, such as a copying machine, a laser printer, and a facsimile device, for supplying one sheet at a time from a stack of sheets.
2. Description of Related Art
There has been known a sheet supply unit for use in various kinds of image forming devices. The sheet supply unit is for supplying one sheet at a time from a stack of sheets to an image forming unit.
As shown in FIGS. 1 and 2, a conventional sheet supply unit 1 includes a sheet supply roller 2, collars 10, a separation pad 3, a pad urging member 4, and guide rollers 12, 13. The sheet supply roller 2 is rotatable in a direction indicated by an arrow A, and is made of a resilient material. The sheet supply roller 2 has a circumferential surface 8 and a retracted surface 9 retracted from the circumferential surface 8 in a radial direction. Shoulders 14a, 15a are formed between the circumferential surface 8 and retracted surface 9.
As shown in FIG. 2, the collars 10 are formed in a disc like shape. The collars 10 are rotatably disposed in a coaxial relation with the sheet supply roller 2 and sandwich the sheet supply roller 2 therebetween. Each collar 10 is formed so that its peripheral surface is retracted radially inwardly from the circumferential surface 8 of the supply roller 2, and protrudes radially outwardly from the retracted surface 9.
The pad urging member 4 is disposed in confrontation with the sheet supply roller 2 and the collars 10. The pad urging member 4 includes a pad holder 5, stoppers 6, and springs 7. The pad holder 5 has a sheet receiving portion 5a at an upper end. The sheet receiving portion 5a has an engraved upper surface where the separation pad 3 is attached. The stoppers 6 are disposed at fixed positions adjacent to a lower end of the pad holder 5. The springs 7 are disposed between the stoppers 6 and the sheet receiving portion 5a for urging the pad holder 5 toward the sheet supply roller 2 and the collars 10.
As shown in FIG. 1, when the circumferential surface 8 is in confrontation with the separation pad 3, the circumferential surface 8 contacts a sheet S interposed between the sheet supply roller 2 and the separation pad 3. At this time, the collars 10 are separated from the sheet S. On the other hand, when the retracted surface 9 is brought into confrontation with the separation pad 3, the collars 10 come into contact with the sheet S, and the sheet supply roller 2 is separated from the sheet S.
The guide rollers 12, 13 are rotatably disposed at a downstream side of the sheet supply roller 2 with respect to a sheet feed direction indicated by an arrow B. The guide rollers 12, 13 contact each other so as to develop a nip portion therebetween.
Next, a sheet supply operation of the above sheet supply unit 1 will be described. First, the retracted surface 9 is in confrontation with the separation pad 3 with a sheet S interposed therebetween. At this time, the sheet S is in contact with the collars 10, and the sheet supply roller 2 is suspended above the sheet S without contacting the sheet S. Then, the sheet supply roller 2 is driven to rotate in the direction A. When the shoulder 15 comes in contact with the sheet S, the collars 10 are separated from the sheet S. When the sheet supply roller 2 further rotates while the circumferential surface 8 is in contact with the sheet S, the sheet S is fed in the feed direction B.
When the shoulder 14 separates from the sheet S, the collars 10 are brought into contact with the sheet S so that the sheet supply roller 2 is kept from contacting the sheet S. The sheet supply roller 2 stops rotating in this state. By this time, a leading edge of the sheet S reaches the nip portion developed between the guide rollers 12, 13. The sheet S is further fed by the rotational movement of the guide rollers 12, 13. Because the collars 10 are in contact with the sheet S, the collars 10 rotate idly in accordance with the feeding movement of the sheet S.
However, the above-described sheet supply roller 2 has some drawbacks. As shown in FIG. 1, a step 15a is formed between the peripheral surface of the collar 10 and the shoulder 15. Because of the step 15a, when the shoulder 15 comes into contact with a sheet S, the sheet S is subject to a sharp increase of pressing force between the shoulder 15 and the separation pad 3. At this moment, an additional sheet may be allowed to enter between the sheet supply roller 2 and the separation pad 3. This causes redundant supply of sheets. Also, the sheet S may creased when strongly pressed by the shoulder 15.
In order to solve the above problems, it is conceivable to form a sheet supply roller in an elliptical shape without a shoulder. However, in this case, a first contact position at which a sheet S comes into contact with the sheet supply roller will be a position retracted from the shoulder 15 in the radial direction. This may cause failure on supplying sheet, that is, no sheet may be supplied.
Also, it is conceivable to provide a cam member to the sheet supply roller 2 so that no shoulder would be formed. However, this may cause failure on supplying sheet also.
Also, a step 14a is formed between the peripheral surface of the collar 10 and the shoulder 14. Because of the step 14a, at the moment a sheet S is separated from the shoulder 14 and brought into contact with the collars 10, the sheet S is subject to no pressing force. This is because the pad urging member 4 cannot respond quickly enough to keep generating constant pressing force on the sheet S. Then, at the moment, an additional sheet may be allowed to enter between the collars 10 and the separation pad 3, thereby causing redundant supply of sheet.